1. Field of the Invention
This invention relates to a process for the manufacture of an antistatic and biaxially oriented polypropylene film which is greatly improved in its antistatic property and in its adhesiveness to ink printed on it and to resins coated on it.
2. Description of the Prior Art
Conventional biaxially oriented polypropylene film possesses excellent optical, electric, and mechanical properties, as well as gas- and water-proofness properties. Such films are widely used, for example, as wrapping and insulating materials, and the like. A disadvantage, however, of such a film is that it tends to generate and accumulate static electric charges so that it emits sparks and gives shocks to workers who handle it. Because of these properties, the processability and printability of the film is reduced and/or dust and dirt stick on its surface, and as a result, the commercial value of the film is reduced.
There are two known methods to avoid such disadvantages. One is to coat an antistatic agent on the surface of the film and the other is to blend an antistatic agent together with the polypropylene resin in the course of making the film. The former method cannot provide a long-lasting effective antistatic property. On the other hand, the latter method is characterized by the fact that the antistatic agent oozes from the inside of the film onto its surface, or blooms, to give the film an antistatic property which remains effective for a long time. In this respect, the latter method is far superior to the former.
When, however, the molecular chains of the polypropylene resin are highly crystalline, the molecular chains in the resulting film accordingly become highly oriented. When this kind of polypropylene resin is used, the latter antistatic treatment method is ineffective to confer a sufficient antistatic effect on the film because the crystalline and oriented molecular structure prevents the antistatic agent from oozing from the inside of the film onto its surface. Therefore, to provide an effective antistatic property, the film must be subjected to a surface-treatment such as a corona discharge treatment, to enhance the migration of the antistatic agent onto its surface.
Further, it is desired to print inks or coat resins onto the surface of biaxially oriented film manufactured of polypropylene resin having highly crystalline molecular chains. During such applications of inks and resins, inherently, static electricity is apt to be generated on the surface of the film and the solvent used in the ink or the resin is apt to ignite. Thus, a film intended for this use is required to have an antistatic property. For this purpose, it is customary to blend an antistatic agent with the polypropylene resin and to subject the film to corona discharge. The difficulty that occurs, however, with biaxially oriented polypropylene film containing an antistatic agent that has been treated by corona discharge, is that the antistatic agent oozes out all over the film surface, thereby creating a condition which adversely affects the adhesiveness of inks and resins to the film surface. Thus, the polypropylene film, when subjected to corona discharge, possesses an increased antistatic property, but its adhesiveness is decreased.